The present invention relates to a method and an apparatus for molding a tubular container and, more particularly, to a method and apparatus for molding such a tubular container that its preformed body is formed as a unitary structure with its conical and neck portion.
A synthetic resin made tubular container is usually formed of pliable resin, such as of the olefine series, and hence has the excellent property of permitting a proper quantity of its contents to be squeezed out very easily by pressing the container body with fingers of a user. Tubular containers of this kind are widely used for liquid or pasty materials.
Generally, there are two types of methods for molding such a tubular container. One method is to mold the body and the conical and neck portions into a unitary structure by extrusion blow molding or injection extrusion molding. The other method is to fuse the conical portion with a preformed body portion by injection molding.
With the former method, however, it is impossible to employ a laminated structure for the tubular body portion to add thereto a decorative touch, and it is difficult to control the transparency of the container itself and to form the tubular container with a uniform thickness. Furthermore, the surface of the tubular container is roughened so that no decoration can be effected by offset printing or hot stamping. Due to such fatal defects, this method is not employed for molding of cosmetics containers that are required to give a beautiful or gorgeous appearance and, at present, it is merely used for miscellaneous goods, for instance, paste containers.
The latter molding method is free from such defects as mentioned above but possesses the following drawback. That is, since the contents of the tubular container, such as liquid or pasty cosmetics or the like, are squeezed out therefrom by pressing the tubular body with the fingers of the user, the container has applied thereto external and internal stresses for compressive deformation and this sometimes causes cracking of the container shoulder portion joining the container body and conical portion, resulting in leakage of the content.
Moreover, the latter molding method is accomplished by the use of a molding apparatus such as shown in FIG. 1, which comprises a cylindrical core block 1 for defining the interior of the container, a cavity mold 4 disposed on top of the core block 1 for forming therebetween a conical portion 2 and a neck portion 3 of the container, and a core clamping block 5. The inner wall of the lower end portion of the container shoulder forming part of the cavity mold 4 has a vertical surface 6 to define a cylindrical space having a diameter a little larger than that of the container body A, and a guide portion 7 of the cavity mold 4 is spread out downwardly from the lower end of the vertical wall 6. For molding the tubular container through the use of this molding apparatus, the core block 1 is inserted into the tubular member A performed by extrusion or the like as the container body, the core block 1, the cavity mold 4 and the core clamping block 5 are positioned as shown in FIG. 1, and resin is injected in the direction of the arrow through a molten resin injecting runner 8 provided at the top of the cavity mold 4, thus molding the container conical and neck portions B and C as a unitary structure with the tubular member A.
With such a molding method, while the molten resin is pressurized for injection from the runner 8 to form the conical portion B and the neck portion C, the tubular body A is merely held between the vertical inner wall surface 6 of the cavity mold 4 and the cylindrical core block 1, so that when the molten resin injection pressure is raised, the molten resin flows into a narrow space between the tubular body A and the core block 1. In this case, if the injected resin flows down the core block 1 nonuniformly, then the lower end of the resin does not become level but forms a wavy curve X as shown in FIG. 2. In the case where the container body A is formed of a transparent or translucent resin, such a wavy curve can be clearly seen from the outside and markedly ruins the appearance of the container, giving the impression that not only the container itself but also its contents are cheap.
With the recent trend to diversification, there is also a strong and growing demand for a tubular container the neck and conical portions of which are opaque but the body of which is transparent or translucent so that the contents can be seen therethrough. In this case, the aforementioned wavy curve formed by the lower end of the injected resin becomes more noticeable, and hence there is a strong demand to avoid such occurrence. For preventing the formation of the wavy curve, there has been proposed a method of injecting a fixed quantity of resin for packing into the cavity mold. With this method, however, there is the strong fear of under-packing of the resin and, further, since the injection pressure cannot be raised high, the adhesive strength between the conical portion and the shoulder portion of the container body is reduced. Moreover, in the case of molding a number of tubular containers at a time, the packing rate and the internal pressure of the resin vary largely, further introducing a factor for unstable molding. Accordingly, the use of such a method prevents the formation of the wavy curve but inevitably leads to nonuniformity in the quality of moldings and lowered productivity.
With such a background, the present invention has been made to solve the abovementioned problems of the prior art.
Therefore, it is an object of the present invention to provide an extremely advantageous tubular container molding method and apparatus with which it is possible, without lowering productivity and impairing the adhesive strength between the shoulder and conical portions of the container, to remove the wavy curve which appreciably ruins its appearance and which is impossible with the conventional molding methods to remove.